Adhesives, often referred to as cements, glues or pastes are defined generally as substances capable of holding materials together by surface attachment. Adhesives may attach to surfaces and develop the internal or cohesive strength necessary to hold the materials together while cooling from liquid to solid state, while losing solvent, or during a chemical reaction. Many of the substances designated as adhesives may also be designated as caulking, potting, sealing, casting or encapsulating compounds when employed in a thick mass. In a more restrictive sense, to be termed an adhesive, a substance should be a liquid or a tacky semi-solid, at least for a short period of time in order to contact and wet a surface, and should be used in a relatively thin layer to form a useful joint capable of transmitting stresses from one substrate to another. Thus, the term “adhesives”, as used in this specification and in the claims, includes cements, glues and pastes.
Solvent cementing is a process in which thermoplastics, usually amorphous, are softened by the application of a suitable solvent or mixture of solvents, and then pressed together to effect a bond. Many thermoplastic substrates are easier to join effectively by solvent cements than by conventional adhesive bonding. Generally, a small amount of the same resin as that to be cemented is dissolved in a solvent to form the cement. The inclusion of the resin aids in gap filling, accelerates setting, and reduces shrinkage and internal stresses.
Solvent cements also have been utilized to bond different plastic materials to each other, but in such instances, the solvent must be a solvent for both plastics. Usually in such instances, a mixture of solvents is used. The solvent softens (dissolves) the surfaces of the resins to be bonded, and the surface becomes tacky. At this point, the surfaces are brought into contact with each other, often under pressure, and dried.
For many years, solvent cements have been used for joining ABS (acrylonitrile-butadiene-styrene), PVC (polyvinylchloride) and CPVC (chlorinated polyvinylchloride) plastic pipe and fittings. The amount of such plastic pipe used for conveying a variety of liquids is enormous. The major uses are drain, waste, vent, sewer and potable water conveyance. Plastic pipe has increasingly displaced the traditional materials used for the same purpose such as copper, steel, galvanized metal, cast iron, lead and concrete asbestos pipe. Plastic pipe has become the material of choice in the home, municipal, and manufacturing industries.
The current ASTM standard which relates to ABS solvent cements is ASTM D-2235 which specifies that the ABS solvent cements will contain a minimum of 15% by weight of ABS resin and the remainder is methyl ethyl ketone (MEK) solvent. To have a useful product, the ABS cement typically utilizes 30% to 35% resin and the remainder is MEK. The purpose of using higher than minimum resin is to produce a product that has a suitable viscosity for application to pipes and fittings. Viscosity is measured in centipoise, and a typical ABS cement will have a viscosity of 1000 to 3000 centipoise.
The ASTM standard for PVC solvent cements is ASTM D-2564. According to this standard, such solvent cements contain a minimum of 10% PVC resin, inert fillers, and the remainder is one or more solvents from the group of tetrahydrofuran (THF), cyclohexanone (CYH), MEK, and acetone (ACE).
ASTM F-493 sets forth the requirements for CPVC solvent cements as containing a minimum of 10% CPVC resin, inert fillers, and the remainder is one or more solvents including THF, CYH, MEK and/or acetone.
Because of the critical function of the cement or adhesives in maintaining the integrity of the piping system subjected to various and extreme conditions, there is a continuing need for improved adhesive compositions and solvent cement systems. There is also a continuing need for adhesive compositions which are safer and effective.